1. Field of the Subject Matter Disclosed
The subject matter disclosed relates to nuclear measurements made in a subsurface environment. More particularly, this patent specification relates to systems and methods for track determination of ionizing radiation for density measurements and/or permanent and/or semi-permanent density monitoring in a borehole.
2. Background of the Subject Matter Disclosed
Nuclear density measurements rely on the attenuation of a radiation field of known intensity traversing a region of interest. Such a transmission measurement technique requires the determination of the radiation field intensity before and after the region of interest. In a down-hole environment density measurements are typically performed in a single cylindrical bore-hole tool. This constraint typically prevents transmission type measurements.
In typical down-hole gamma-gamma density measurements, the bore-hole tool contains both one or more photon detectors and a gamma radiation source of known intensity. The formation density of interest is probed when photons emitted from the radiation source exit the bore-hole region and scatter off the formation back into the bore-hole again where they are detected by the density tool. In the energy range of practical down-hole gamma radiation sources, the above photon flux is very sensitive to a variety of environmental effects at the interface between the borehole, the tool and the formation, which affect the measured apparent formation density and must be corrected for.
Typical down-hole photon detectors used in gamma-gamma density measurements consist of a bulk of scintillating material in a suitable package and optically coupled to a photo-multiplier tube: this arrangement measures the energy deposited in the detector by an incoming photon or gamma ray but does not determine the trajectory with which the photon entered the detection volume.
Another type of density measurements uses so-called natural cosmic ray radiation, consisting of charged particles (muons) up to very high energies. Cosmic ray muons can penetrate up to several km into the subsurface and are thus of interest for subsurface density measurements probing formation densities at much larger distance scales than with gamma-gamma density and up to several tens of meters or more. However given the modest cosmic muon rate (only a few events per square inch per minute at surface, e.g., a surface of the earth, and dropping exponentially with depth) it becomes quickly unpractical to perform a muon transmission measurement over long ranges as the usable flux will be quickly significantly reduced by the practical solid angle coverage available when two detectors are placed over long distances.